The proposed research involves structural studies with an enzyme from the reductive acetyl-CoA pathway for CO 2 fixation. Clostridium thermoaceticum, an anaerobic acetogene, is the model system for this unusual pathway. MeTr transfers the N -methyl group of (6S)-methyltetrahydrofolate to the cobalt center of a corrinoid/iron sulfur protein. Determining this MeTr structure will also shed light on the structure and function of other closely related cobalamin-dependent methyltransferases: those from methanogenic bacteria and the methyl transfer in the first half reaction of B 12 -dependant methionine synthase (MeS) enzymes.